Update minimum Tornado version to 6.5.0 to address:
- CVE-2024-52804 (Medium): HTTP Cookie Parsing DoS
- CVE-2025-47287 (High 7.5): Multipart/Form-Data Parser DoS
This simplifies the tornado worker by removing legacy code paths
for Tornado < 5.0 and < 6.0, reducing the codebase by ~30%.
Changes:
- pyproject.toml: Update tornado requirement to >=6.5.0
- gtornado.py: Remove TORNADO5 constant and legacy code paths
- tornadoapp.py: Update example to use async/await syntax
- test_gtornado.py: Add comprehensive test suite
